An available RF transmitter allows a user to listen to a headphone-only portable audio unit (such as a portable stereo CD or tape player) through a radio receiver especially in the situations where headphones are not allowed or impractical, such as listening in a moving vehicle.
Currently known state of the art RF transmitters, such as an FM transmitter, intended for this purpose require a user in the vehicle to tune both the transmitter and the radio receiver. This approach is exemplified by U.S. Pat. No. 5,319,716 to William T. McGreevy, assigned to Recoton Corporation. McGreevy's invention involves a vehicle's passenger tuning the transmitter of an adapter to the same frequency as that of the tuner of the vehicle's FM radio receiver system.
However, tuning the transmitter in a moving vehicle could cause a serious safety concern especially when the user is the person who is driving. Safe operation of the vehicle could be jeopardized by a mental and physical distraction of the driver whenever he attempts to tune the transmitter. Moreover, it would not be easy for the driver to physically locate and tune the transmitter since, unlike a firmly installed car radio receiver, the transmitter is usually positioned unsteadily in the passenger compartment of the vehicle, together with a portable audio unit.
It is noted that retuning the transmitter may be necessary as the vehicle movers from one location to anther. This is because an open radio channel on which the transmitter is initially tuned in a particular geographical region may not be open (i.e., occupied by the local broadcast station) in another geographical region. Therefore, McGreevy's invention creates a possible safety problem for automobile users.